Dosimeter formed of a radiation sensitive thermoluminescent material and method of reading the same

ABSTRACT

A dosimeter incorporates a crystalline thermoluminescent structure to store radiant energy. The crystalline thermoluminescent material can be a combination of different materials or a single material. The dosimeter is read out by applying energy to the crystal to cause mechanical vibration or crystal lattice vibration at the natural resonant frequency of the crystal. The energy applied may be an alternating current or coherent light energy as desired.

United States Patent Inventors Jacob Kastner Downers Grove;

Billie G. OItman, Worth, both of 111. 856,278

Sept. 9, 1969 Oct. 5, 1971 The United States of America as representedby the United States Atomic Energy Commission Appl. No. Filed PatentedAssignee DOSIMETER FORMED OF A RADIATION SENSITIVE THERMOLUMINESCENTMATERIAL AND METHOD OF READING THE SAME OTHER REFERENCESThermoluminescence-Theory & Applications by Lancaster- ElectronicsWorld, Mar. 1969, pp. 43- 46.

Primary Examiner.lames W. Lawrence Assistant Examiner-P. C. NelmsAttorney-Roland A. Anderson ABSTRACT: A dosimeter incorporates acrystalline thermoluminescent structure to store radiant energy. Thecrystal- 6 Claims 5 Drawing Figs line thermoluminescent material can bea combination of dif US. Cl 250/71, ferent materials or a singlematerial. The dosimeter is read out 250/71.5 by applying energy to thecrystal to cause mechanical vibra- Int. Cl ..G0ln 23/00 tion or crystallattice vibration at the natural resonant Field of Search 250/71 R,frequency of the crystal. The energy applied may be an alter- 71.5 Rnating current or coherent light energy as desired.

6? PHoro MULT/PL/ER fiMp/JF/ER POWER su TUBE RECORDER A MI /6 k\\\\\ l571 6 E IVER/1 7 0A DOSIME'IER FORMED OF A RADIATION SENSITIVETIIERMOLUMINESCENT MATERIAL AND METHOD OF READING THE SAME CONTRACTUALORIGIN OF THE INVENTION The invention described herein was made in thecourse of, or under, a contract with the United States Atomic EnergyCommission.

BACKGROUND OF THE INVENTION A standard technique for radiation dosimetryinvolves the thermal stimulation of luminescence by applying heat energyto an irradiated thermoluminescent material directly from a heat source.It is desirable to improve this technique in order to measure the storedenergy in the thermoluminescent material more easily. It is also usefulto have a thermoluminescent dosimeter which can be read without heating.Preferably the dosimeter could be constructed in the form of a badge ormedallion which could be positioned as desired.

It is therefore an object of this invention to provide an improvedthermoluminescent dosimeter.

Another object of this invention is to provide a thermoluminescentdosimeter which can be read without the direct application of thermalenergy.

Another object of this invention is to provide a thermoluminescentdosimeter which can be read without heating the thermoluminescentmaterial.

SUMMARY OF THE INVENTION In practicing this invention a dosimeter isprovided in which the energy storage material is a radiation-sensitivecrystalline thermoluminescent material. The radiation-sensitive materialcan consist of a plate of piezoelectric material with theradiation-sensitive material disposed thereon, a plate of piezoelectricmaterial, such as quartz, which is also thermoluminescent, or a plate ofradiation-sensitive thermoluminescent crystalline material such as CaFor LiF.

After irradiation of the radiation-sensitive material it can be read bysupplying sufiicient energy at the electron trap sites to allow thestored energy to be released as luminescence. In prior art devices thisenergy was supplied by directly heating the radiation-sensitivematerial. In the structure of this invention the energy is supplied byindirectly heating the radiationsensitive material or, in particularcases, supplying energy directly to the crystal lattice to cause thelattice to vibrate at its natural resonant frequency.

In one embodiment of the invention, using a piezoelectric plate, readoutis accomplished by the application of energy to the piezoelectric plateso that the plate vibrates mechanically. The mechanical vibration of thepiezoelectric crystal develops heat which causes the stored energy to bereleased from the radiation-sensitive material. This method isapplicable to either the structure where the piezoelectric material isalso thermoluminescent or where the thermoluminescent material isseparate from the piezoelectric material.

In the structure where a crystalline thermoluminescent material is used,stored energy release can be obtained without heating if the frequencyof the applied energy is suffrciently high to cause lattice vibration atthe natural crystal frequency. One method of accomplishing this is tosupply coherent light energy from a laser to the crystal. For example,light energy from a ruby laser at a wavelength of approximately 6,900angstroms will cause lattice vibration of a quartz crystal at itsnatural resonant frequency of the order of IO" c.p.s., due to thewell-known Raman effect. In the Raman effect a small portion of theenergy in the laser beam acts to cause lattice vibration of thethermoluminescent crystal. The resulting lattice vibration will causethe stored energy to be released as light.

When the dosimeter is read by means of a laser beam thethermoluminescent material does not need to be piezoelectric as thelaser beam will cause crystal lattice vibration of the crystallinematerial even if symmetrical. Thus a thermoluminescent material, such asCaF or LiF, can be used. The crystalline material, however, must betransparent to the radiation from the laser beam. By transparent it ismeant that the energy extracted from the laser beam and resulting incrystal heating must be appreciably less than the energy extracted fromthe laser beam and resulting in crystal lattice vibration. A

BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustrated in thedrawings of which:

FIG. 1 is a cross section of the dosimeter of this invention; dosimeter;

FIG. 2 is a cross section of a second embodiment of the dosimeter;

FIG. 3 is a cross section of a third embodiment of the dosimeter;

FIG. 4 illustrates the method by which the energy stored in thedosimeter of FIGS. 1 and 3 is read out; and 7 FIG. 5 illustrates themethod by which the energy stored in the dosimeter of FIG. 2 is readout.

DETAILED DESCRIPTION OF THE INVENTION In FIG. I there is shown adosimeter in the fonn of a badge or medallion incorporating the featuresof this invention. The dosimeter comprises an enclosure 10 whichcontains a piezoelectric crystal 11 mounted therein. The piezoelectriccrystal 11 has electrodes 15 and I6 mounted on opposite faces,

thereof. Wire 19 extends from electrode 16 to contact 21 and wire 20extends from electrode 15 to contact 22.

Disposed over the face of piezoelectric crystal II is athermoluminescent material 12 which may be, for example, LiF or CaF. Atransparent plastic window I4 acts to seal the badge and is used forreadout of the stored energy.

Fln FIG. 2 there is shown another form of the badge which incorporates asingle layer of thermoluminescent material. The enclosure 25 supportsthe thermoluminescent material 28. The thermoluminescent material may bea single layer of piezoelectric material which is alsothermoluminescent, such as quartz, or it may be a thermoluminescentmaterial such as CaF or LiF. First and second transparent windows 26 and29 act to seal the badge and provide means for reading out the storedenergy in a manner to be subsequently described.

In FIG. 3 there is shown the dosimeter structure of FIG. 2 withelectrodes shown in the dosimeter of FIG. I. An enclo sure 31 contains apiezoelectric crystal 30 which is also thermoluminescent. A transparentwindow 32 acts to seal the badge. Electrodes 33 and 34 are positioned onopposite ends of piezoelectric crystal 30'for applying electric energyto the crystal. Electrode 33 is connected to terminal 37 by wire 38 andelectrode 34 is connected to terminal 35 by wire 36.

In FIG. 4 there is shown the method by which the energy stored in thedosimeter as a result of irradiation is measured. After irradiation,dosimeter I0 of FIG. I is placed in a lighttight enclosure 46. Contacts21 and 22 are connected to contacts 39 and 41 respectively. Contacts 39and 41 are connected to signal generator 40 which supplies an electriccurrent of a desired frequency to the electrodes 15 and I6 of dosimeter10, causing mechanical vibration of the piezoelectric crystal 11. Themechanical vibration of crystal ll develops sufficient heat to applyenough energy to the electron trap sites of the thermoluminescentmaterial l2 to release the energy stored therein as luminescence.

While the dosimeter of FIG. I is shown in the readout structure of FIG.4, the dosimeter structure of FIG. 3 could also be used. Dosimeter 31would be placed in the lighttight enclosure 46 with contacts 35 and 37connected to contacts 39 and 41 respectively. Crystal 30 would bemechanically vibrated by the signal applied to electrodes 33 and 34 torelease the energy stored in the piezoelectric thermoluminescent crystalas luminescence.

In FIG. 5 there is shown the method of reading the dosimeter of FIG. 2.Dosimeter 25 is placed in lighttight enclosure 53 for readout. Energy issupplied to the thermoluminescent crystal 28 by means of a beam of lightfrom laser 47. Laser 47 may be, for example, a ruby laser emittingcoherent light at a wavelength of 6,900 angstroms. The light from laser47 passes through the transparent window 26 in dosimeter and suppliesenergy to crystal 28 so that the crystal lattice vibrates at its naturalresonant frequency. This reaction of crystal 28 to the laser beam is thewell-known Raman effect.

The vibration of the lattice of crystal 28 supplies enough energy at theelectron trap sites to allow the stored energy to be released asluminescence as previously described. The luminescence is detected byphotomultiplier 49 and the resulting signal is amplified in amplifier 51and recorded in recorder 52.

It will be understood that the invention is not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A radiation dosimeter including in combination, a radiation-sensitivepiezoelectric thermoluminescent plate having a first portion formed of apiezoelectric material and a second portion formed of aradiation-sensitive thermoluminescent material disposed over saidpiezoelectric material, means attached to said piezoelectric materialfor supporting said plate whereby said thermoluminescent materialreceives radiation and means for indicating the radiation dosage.

2. The radiation dosimeter of claim 1 wherein, said radiation-sensitivethermoluminescent material is UP.

3. The radiation dosimeter of claim 1 wherein, said radia tionsensitivethermoluminescent material is CaF.

4. A method of measuring radiation including the steps of:

a. exposing a radiation-sensitive piezoelectric thermoluminescentcrystal material to the radiation to store energy therein,

b. exciting said material electrically to cause mechanical vibrationthereof to release said stored energy as a light output, and

c. measuring the intensity of said light output.

5. The method of measuring radiation of claim 4, wherein, saidradiation-sensitive piezoelectric thermoluminescent material includes afirst portion formed of a piezoelectric material and a second portionformed of a radiation-sensitive thermoluminescent material, said firstportion being excited electrically to cause mechanical vibrationthereof, said mechanical vibration of said first portion causingmechanical vibration of said second portion to release as a light outputsaid energy stored in said second portion.

6. A method of measuring radiation including the steps of:

a. exposing a radiation-sensitive thermoluminescent crystal material tothe radiation to store energy therein,

b. irradiating said material with a coherent light beam to cause thecrystal lattice of said crystal material to vibrate at its naturalresonant frequency to thereby release said stored energy as a lightoutput, and

c. measuring the intensity of said light output.

1. A radiation dosimeter including in combination, a radiationsensitivepiezoelectric thermoluminescent plate having a first portion formed of apiezoelectric material and a second portion formed of aradiation-sensitive thermoluminescent material disposed over saidpiezoelectric material, means attached to said piezoelectric materialfor supporting said plate whereby said thermoluminescent materialreceives radiation and means for indicating the radiation dosage.
 2. Theradiation dosimeter of claim 1 wherein, said radiation-sensitivethermoluminescent material is LiF.
 3. The radiation dosimeter of claim 1wherein, said radiation-sensitive thermoluminescent material is CaF. 4.A method of measuring radiation including the steps of: a. exposing aradiation-sensitive piezoelectric thermoluminescent crystal material tothe radiation to store energy therein, b. exciting said materialelectrically to cause mechanical vibration thereof to release saidstored energy as a light output, and c. measuring the intensity of saidlight output.
 5. The method of measuring radiation of claim 4, wherein,said radiation-sensitive piezoelectric thermoluminescent materialincludes a first portion formed of a piezoelectric material and a secondportion formed of a radiation-sensitive thermoluminescent material, saidfirst portion being excited electrically to cause mechanical vibrationthereof, said mechanical vibration of said first portion causingmechanical vibration of said second portion to release as a light outputsaid energy stored in said second portion.
 6. A method of measuringradiation including the steps of: a. exposing a radiation-sensitivethermoluminescent crystal material to the radiation to store energytherein, b. irradiating said material with a coherent light beam tocause the crystal lattice of said crystal material to vibrate at itsNatural resonant frequency to thereby release said stored energy as alight output, and c. measuring the intensity of said light output.